Polish "Add support for removing build plugins"

See gh-978

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/build/maven/DefaultMavenBuildCustomizerTests.java

@@ -53,8 +53,7 @@ class DefaultMavenBuildCustomizerTests {
 	void customizeRegisterSpringBootPlugin() {
 		InitializrMetadata metadata = InitializrMetadataTestBuilder.withDefaults().build();
 		MavenBuild build = customizeBuild(metadata);
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((mavenPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((mavenPlugin) -> {
 			assertThat(mavenPlugin.getGroupId()).isEqualTo("org.springframework.boot");
 			assertThat(mavenPlugin.getArtifactId()).isEqualTo("spring-boot-maven-plugin");
 			assertThat(mavenPlugin.getVersion()).isNull();

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/code/groovy/GroovyGradleBuildCustomizerTests.java

@@ -32,8 +32,8 @@ class GroovyGradleBuildCustomizerTests {
 	void groovyPluginIsConfigured() {
 		GradleBuild build = new GradleBuild();
 		new GroovyGradleBuildCustomizer().customize(build);
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((plugin) -> assertThat(plugin.getId()).isEqualTo("groovy"));
+		assertThat(build.plugins().values())
+				.hasOnlyOneElementSatisfying((plugin) -> assertThat(plugin.getId()).isEqualTo("groovy"));
 	}
 
 }

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/code/groovy/GroovyMavenBuildCustomizerTests.java

@@ -34,8 +34,7 @@ class GroovyMavenBuildCustomizerTests {
 	void groovyMavenPluginIsConfigured() {
 		MavenBuild build = new MavenBuild();
 		new GroovyMavenBuildCustomizer().customize(build);
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((groovyPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((groovyPlugin) -> {
 			assertThat(groovyPlugin.getGroupId()).isEqualTo("org.codehaus.gmavenplus");
 			assertThat(groovyPlugin.getArtifactId()).isEqualTo("gmavenplus-plugin");
 			assertThat(groovyPlugin.getVersion()).isEqualTo("1.6.3");

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/code/kotlin/KotlinJpaGradleBuildCustomizerTests.java

@@ -19,6 +19,7 @@ package io.spring.initializr.generator.spring.code.kotlin;
 import java.util.Collections;
 
 import io.spring.initializr.generator.buildsystem.gradle.GradleBuild;
+import io.spring.initializr.generator.buildsystem.gradle.GradlePlugin;
 import io.spring.initializr.generator.buildsystem.gradle.StandardGradlePlugin;
 import io.spring.initializr.generator.spring.test.InitializrMetadataTestBuilder;
 import io.spring.initializr.generator.version.Version;
@@ -41,9 +42,8 @@ class KotlinJpaGradleBuildCustomizerTests {
 		Dependency dependency = Dependency.withId("foo");
 		dependency.setFacets(Collections.singletonList("jpa"));
 		GradleBuild build = getCustomizedBuild(dependency);
-		assertThat(build.plugins().values().filter((plugin) -> plugin.isApply())).isEmpty();
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((plugin) -> {
+		assertThat(build.plugins().values().filter(GradlePlugin::isApply)).isEmpty();
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((plugin) -> {
 			assertThat(plugin.getId()).isEqualTo("org.jetbrains.kotlin.plugin.jpa");
 			assertThat(((StandardGradlePlugin) plugin).getVersion()).isEqualTo("1.2.70");
 		});
@@ -53,7 +53,7 @@ class KotlinJpaGradleBuildCustomizerTests {
 	void customizeWhenJpaFacetAbsentShouldNotAddKotlinJpaPlugin() {
 		Dependency dependency = Dependency.withId("foo");
 		GradleBuild build = getCustomizedBuild(dependency);
-		assertThat(build.plugins().values().filter((plugin) -> plugin.isApply())).isEmpty();
+		assertThat(build.plugins().values().filter(GradlePlugin::isApply)).isEmpty();
 		assertThat(build.plugins().values()).isEmpty();
 	}
 

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/code/kotlin/KotlinJpaMavenBuildCustomizerTests.java

@@ -42,8 +42,7 @@ class KotlinJpaMavenBuildCustomizerTests {
 		Dependency dependency = Dependency.withId("foo");
 		dependency.setFacets(Collections.singletonList("jpa"));
 		MavenBuild build = getCustomizedBuild(dependency);
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((plugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((plugin) -> {
 			assertThat(plugin.getGroupId()).isEqualTo("org.jetbrains.kotlin");
 			assertThat(plugin.getArtifactId()).isEqualTo("kotlin-maven-plugin");
 			MavenPlugin.Setting settings = plugin.getConfiguration().getSettings().get(0);
@@ -61,7 +60,7 @@ class KotlinJpaMavenBuildCustomizerTests {
 	void customizeWhenJpaFacetAbsentShouldNotAddKotlinJpaPlugin() {
 		Dependency dependency = Dependency.withId("foo");
 		MavenBuild build = getCustomizedBuild(dependency);
-		assertThat(build.plugins().values()).hasSize(0);
+		assertThat(build.plugins().isEmpty()).isTrue();
 	}
 
 	private MavenBuild getCustomizedBuild(Dependency dependency) {

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/code/kotlin/KotlinMavenBuildCustomizerTests.java

@@ -54,8 +54,7 @@ class KotlinMavenBuildCustomizerTests {
 	void kotlinMavenPluginIsConfigured() {
 		MavenBuild build = new MavenBuild();
 		new KotlinMavenBuildCustomizer(new SimpleKotlinProjectSettings("1.2.70")).customize(build);
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((kotlinPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((kotlinPlugin) -> {
 			assertThat(kotlinPlugin.getGroupId()).isEqualTo("org.jetbrains.kotlin");
 			assertThat(kotlinPlugin.getArtifactId()).isEqualTo("kotlin-maven-plugin");
 			assertThat(kotlinPlugin.getVersion()).isNull();
@@ -85,8 +84,7 @@ class KotlinMavenBuildCustomizerTests {
 	void kotlinMavenPluginWithSeveralArgs() {
 		MavenBuild build = new MavenBuild();
 		new KotlinMavenBuildCustomizer(new TestKotlinProjectSettings()).customize(build);
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((kotlinPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((kotlinPlugin) -> {
 			Configuration configuration = kotlinPlugin.getConfiguration();
 			Setting args = configuration.getSettings().get(0);
 			assertThat(args.getName()).isEqualTo("args");

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/code/kotlin/KotlinMavenFullBuildCustomizerTests.java

@@ -52,8 +52,7 @@ class KotlinMavenFullBuildCustomizerTests {
 	void kotlinMavenPluginIsConfigured() {
 		MavenBuild build = new MavenBuild();
 		new KotlinMavenFullBuildCustomizer(new SimpleKotlinProjectSettings("1.2.70")).customize(build);
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((kotlinPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((kotlinPlugin) -> {
 			assertThat(kotlinPlugin.getGroupId()).isEqualTo("org.jetbrains.kotlin");
 			assertThat(kotlinPlugin.getArtifactId()).isEqualTo("kotlin-maven-plugin");
 			assertThat(kotlinPlugin.getVersion()).isEqualTo("${kotlin.version}");

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/gradle/GradleBuild.java

@@ -46,6 +46,8 @@ public class GradleBuild extends Build {
 
 	private final Map<String, String> ext = new TreeMap<>();
 
+	private final GradlePluginContainer plugins = new GradlePluginContainer();
+
 	private final List<String> configurations = new ArrayList<>();
 
 	private final Map<String, ConfigurationCustomization> configurationCustomizations = new LinkedHashMap<>();
@@ -58,8 +60,6 @@ public class GradleBuild extends Build {
 
 	private final Buildscript buildscript = new Buildscript();
 
-	private final GradlePluginContainer plugins = new GradlePluginContainer();
-
 	public GradleBuild(BuildItemResolver buildItemResolver) {
 		super(buildItemResolver);
 	}

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/gradle/GradleBuildWriter.java

@@ -82,6 +82,11 @@ public abstract class GradleBuildWriter {
 
 	protected abstract void writePlugins(IndentingWriter writer, GradleBuild build);
 
+	protected List<StandardGradlePlugin> extractStandardPlugin(GradleBuild build) {
+		return build.plugins().values().filter(StandardGradlePlugin.class::isInstance)
+				.map(StandardGradlePlugin.class::cast).collect(Collectors.toList());
+	}
+
 	protected abstract void writeJavaSourceCompatibility(IndentingWriter writer, GradleBuild build);
 
 	protected abstract void writeConfigurations(IndentingWriter writer, GradleBuild build);
@@ -168,11 +173,6 @@ public abstract class GradleBuildWriter {
 		}
 	}
 
-	protected List<StandardGradlePlugin> extractStandardPlugin(GradleBuild build) {
-		return (List<StandardGradlePlugin>) (List<? extends GradlePlugin>) build.plugins().values()
-				.filter((plugin) -> plugin instanceof StandardGradlePlugin).collect(Collectors.toList());
-	}
-
 	private void writeBoms(IndentingWriter writer, GradleBuild build) {
 		if (build.boms().isEmpty()) {
 			return;

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/gradle/GradlePlugin.java

@@ -25,7 +25,7 @@ public class GradlePlugin {
 
 	private final String id;
 
-	private boolean apply;
+	private final boolean apply;
 
 	public GradlePlugin(String id, boolean apply) {
 		this.id = id;

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/gradle/GradlePluginContainer.java

@@ -29,11 +29,7 @@ import java.util.stream.Stream;
  */
 public class GradlePluginContainer {
 
-	private final Map<String, GradlePlugin> plugins;
-
-	public GradlePluginContainer() {
-		this.plugins = new LinkedHashMap<>();
-	}
+	private final Map<String, GradlePlugin> plugins = new LinkedHashMap<>();
 
 	/**
 	 * Specify if this container is empty.
@@ -44,16 +40,16 @@ public class GradlePluginContainer {
 	}
 
 	/**
-	 * Specify if this container has a GradlePlugin with the specified id.
-	 * @param id id associated with the {@link GradlePlugin}
-	 * @return {@code true} if an object with the specified id is added
+	 * Specify if this container has a plugin with the specified id.
+	 * @param id the identifier of a gradle plugin
+	 * @return {@code true} if a plugin with the specified {@code id} exists
 	 */
 	public boolean has(String id) {
 		return this.plugins.containsKey(id);
 	}
 
 	/**
-	 * Returns a {@link Stream} of added {@link GradlePlugin}s.
+	 * Returns a {@link Stream} of registered {@link GradlePlugin}s.
 	 * @return a stream of {@link GradlePlugin}s
 	 */
 	public Stream<GradlePlugin> values() {
@@ -61,41 +57,49 @@ public class GradlePluginContainer {
 	}
 
 	/**
-	 * Add a {@link GradlePlugin} to the Gradle's legacy apply block by specifying the id.
-	 * @param id id associated with the {@link GradlePlugin}
-	 */
-	public void apply(String id) {
-		addPlugin(id, (pluginId) -> new GradlePlugin(pluginId, true));
-	}
-
-	/**
-	 * Add a {@link GradlePlugin} to the Gradle's standard plugins DSL block by specifying
-	 * the id.
-	 * @param id id associated with the {@link GradlePlugin}
+	 * Add a {@link GradlePlugin} to the standard {@code plugins} block with the specified
+	 * id. Does nothing if the plugin has already been added.
+	 * @param id the id of the plugin
+	 * @see #add(String, Consumer)
 	 */
 	public void add(String id) {
-		addPlugin(id, (pluginId) -> new StandardGradlePlugin(pluginId));
+		addPlugin(id, StandardGradlePlugin::new);
 	}
 
 	/**
-	 * Add a {@link GradlePlugin} to the Gradle's standard plugins DSL block by specifying
-	 * the id, along with a {@link Consumer} to customize the object.
-	 * @param id id associated with the {@link GradlePlugin}
-	 * @param plugin consumer to customize the {@link GradlePlugin}
+	 * Add a {@link GradlePlugin} to the standard {@code plugins} block with the specified
+	 * id and {@link Consumer} to customize the object. If the plugin has already been
+	 * added, the consumer can be used to further tune the existing plugin configuration.
+	 * @param id the id of the plugin
+	 * @param plugin a {@link Consumer} to customize the {@link GradlePlugin}
 	 */
 	public void add(String id, Consumer<StandardGradlePlugin> plugin) {
-		GradlePlugin gradlePlugin = addPlugin(id, (pluginId) -> new StandardGradlePlugin(pluginId));
+		GradlePlugin gradlePlugin = addPlugin(id, StandardGradlePlugin::new);
 		if (gradlePlugin instanceof StandardGradlePlugin) {
 			plugin.accept((StandardGradlePlugin) gradlePlugin);
 		}
 	}
 
-	public boolean remove(String id) {
-		return this.plugins.remove(id) != null;
+	/**
+	 * Apply a {@link GradlePlugin} with the specified id. Does nothing if the plugin has
+	 * already been applied.
+	 * @param id the id of the plugin
+	 */
+	public void apply(String id) {
+		addPlugin(id, (pluginId) -> new GradlePlugin(pluginId, true));
 	}
 
 	private GradlePlugin addPlugin(String id, Function<String, GradlePlugin> pluginId) {
 		return this.plugins.computeIfAbsent(id, pluginId);
 	}
 
+	/**
+	 * Remove the plugin with the specified {@code id}.
+	 * @param id the id of the plugin to remove
+	 * @return {@code true} if such a plugin was registered, {@code false} otherwise
+	 */
+	public boolean remove(String id) {
+		return this.plugins.remove(id) != null;
+	}
+
 }

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/gradle/GroovyDslGradleBuildWriter.java

@@ -76,7 +76,7 @@ public class GroovyDslGradleBuildWriter extends GradleBuildWriter {
 	}
 
 	private List<GradlePlugin> extractApplyPlugins(GradleBuild build) {
-		return build.plugins().values().filter((plugin) -> plugin.isApply()).collect(Collectors.toList());
+		return build.plugins().values().filter(GradlePlugin::isApply).collect(Collectors.toList());
 	}
 
 	private String pluginAsString(StandardGradlePlugin plugin) {

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/gradle/KotlinDslGradleBuildWriter.java

@@ -67,16 +67,12 @@ public class KotlinDslGradleBuildWriter extends GradleBuildWriter {
 	protected void writePlugins(IndentingWriter writer, GradleBuild build) {
 		writeNestedCollection(writer, "plugins", extractStandardPlugin(build), this::pluginAsString, null);
 		writer.println();
-		if (checkForApplyPlugins(build)) {
+		if (build.plugins().values().anyMatch(GradlePlugin::isApply)) {
 			throw new IllegalStateException(
 					"build.gradle.kts scripts shouldn't apply plugins. They should use the plugins block instead.");
 		}
 	}
 
-	private boolean checkForApplyPlugins(GradleBuild build) {
-		return build.plugins().values().anyMatch((gradlePlugin) -> gradlePlugin.isApply());
-	}
-
 	private String pluginAsString(StandardGradlePlugin plugin) {
 		String result = shortPluginNotation(plugin.getId());
 		if (result == null) {

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/gradle/StandardGradlePlugin.java

@@ -16,6 +16,11 @@
 
 package io.spring.initializr.generator.buildsystem.gradle;
 
+/**
+ * A standard {@link GradlePlugin} used in the {@code plugins} block.
+ *
+ * @author HaiTao Zhang
+ */
 public class StandardGradlePlugin extends GradlePlugin {
 
 	private String version;

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/maven/MavenPluginContainer.java

@@ -28,11 +28,7 @@ import java.util.stream.Stream;
  */
 public class MavenPluginContainer {
 
-	private final Map<String, MavenPlugin> plugins;
-
-	public MavenPluginContainer() {
-		this.plugins = new LinkedHashMap<>();
-	}
+	private final Map<String, MavenPlugin> plugins = new LinkedHashMap<>();
 
 	/**
 	 * Specify if this container is empty.
@@ -43,19 +39,19 @@ public class MavenPluginContainer {
 	}
 
 	/**
-	 * Specify if this container has a MavenPlugin with the specified groupId and
-	 * artifactId.
-	 * @param groupId groupId associated with the {@link MavenPlugin}
-	 * @param artifactId artifactId associated with the {@link MavenPlugin}
+	 * Specify if this container has a plugin with the specified {@code groupId} and
+	 * {@code artifactId}.
+	 * @param groupId the groupId of the plugin
+	 * @param artifactId the artifactId of the plugin
 	 * @return {@code true} if an item with the specified {@code groupId} and
-	 * {@code artifactId} is added
+	 * {@code artifactId} exists
 	 */
 	public boolean has(String groupId, String artifactId) {
 		return this.plugins.containsKey(pluginKey(groupId, artifactId));
 	}
 
 	/**
-	 * Returns a {@link Stream} of added {@link MavenPlugin}s.
+	 * Returns a {@link Stream} of registered {@link MavenPlugin}s.
 	 * @return a stream of {@link MavenPlugin}s
 	 */
 	public Stream<MavenPlugin> values() {
@@ -63,31 +59,39 @@ public class MavenPluginContainer {
 	}
 
 	/**
-	 * Add a {@link MavenPlugin} by specifying the groupId and artifactId.
-	 * @param groupId groupId associated with the {@link MavenPlugin}
-	 * @param artifactId artifactId associated with the {@link MavenPlugin}
+	 * Add a {@link MavenPlugin} with the specified {@code groupId} and
+	 * {@code artifactId}. Does nothing if the plugin has already been added.
+	 * @param groupId the groupId of the plugin
+	 * @param artifactId the artifactId of the plugin
+	 * @see #add(String, String, Consumer)
 	 */
 	public void add(String groupId, String artifactId) {
 		addPlugin(groupId, artifactId);
 	}
 
 	/**
-	 * Add a {@link MavenPlugin} by specifying the groupId and artifactId, along with a
-	 * {@link Consumer} to customize the object.
-	 * @param groupId groupId associated with the {@link MavenPlugin}
-	 * @param artifactId artifactId associated with the {@link MavenPlugin}
-	 * @param plugin {@link Consumer} to customize the object
+	 * Add a {@link MavenPlugin}with the specified {@code groupId} and {@code artifactId}
+	 * and {@link Consumer} to customize the object. If the plugin has already been *
+	 * added, the consumer can be used to further tune the existing plugin configuration.
+	 * @param groupId the groupId of the plugin
+	 * @param artifactId the artifactId of the plugin
+	 * @param plugin a {@link Consumer} to customize the {@link MavenPlugin}
 	 */
 	public void add(String groupId, String artifactId, Consumer<MavenPlugin> plugin) {
 		MavenPlugin mavenPlugin = addPlugin(groupId, artifactId);
 		plugin.accept(mavenPlugin);
 	}
 
+	private MavenPlugin addPlugin(String groupId, String artifactId) {
+		return this.plugins.computeIfAbsent(pluginKey(groupId, artifactId),
+				(pluginId) -> new MavenPlugin(groupId, artifactId));
+	}
+
 	/**
-	 * Remove a {@link MavenPlugin} by specifying the groupId and artifactId.
-	 * @param groupId groupId associated with the {@link MavenPlugin}
-	 * @param artifactId artifactId associated with the {@link MavenPlugin}
-	 * @return {@code true} if an object was removed
+	 * Remove the plugin with the specified {@code groupId} and {@code artifactId}.
+	 * @param groupId the groupId of the plugin to remove
+	 * @param artifactId the artifactId of the plugin to remove
+	 * @return {@code true} if such a plugin was registered, {@code false} otherwise
 	 */
 	public boolean remove(String groupId, String artifactId) {
 		return this.plugins.remove(pluginKey(groupId, artifactId)) != null;
@@ -97,9 +101,4 @@ public class MavenPluginContainer {
 		return String.format("%s:%s", groupId, artifactId);
 	}
 
-	private MavenPlugin addPlugin(String groupId, String artifactId) {
-		return this.plugins.computeIfAbsent(pluginKey(groupId, artifactId),
-				(pluginId) -> new MavenPlugin(groupId, artifactId));
-	}
-
 }

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/gradle/GradlePluginContainerTests.java

@@ -24,40 +24,67 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link GradlePluginContainer}.
  *
  * @author HaITao Zhang
+ * @author Stephane Nicoll
  */
-public class GradlePluginContainerTests {
+class GradlePluginContainerTests {
 
 	@Test
-	void addGradlePluginWithOnlyId() {
+	void addPluginWithId() {
 		GradlePluginContainer pluginContainer = new GradlePluginContainer();
 		pluginContainer.add("com.example");
-		assertThat(pluginContainer.values()).hasSize(1);
-		pluginContainer.values().findFirst().ifPresent((plugin) -> {
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
 			assertThat(plugin.getId()).isEqualTo("com.example");
 			assertThat(plugin.isApply()).isFalse();
 		});
 	}
 
 	@Test
-	void addGradlePluginWithConsumer() {
+	void addPluginWithConsumer() {
 		GradlePluginContainer pluginContainer = new GradlePluginContainer();
 		pluginContainer.add("com.example", (plugin) -> plugin.setVersion("1.0"));
-		assertThat(pluginContainer.values()).hasSize(1);
-		pluginContainer.values().findFirst().ifPresent((plugin) -> {
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
 			assertThat(plugin.getId()).isEqualTo("com.example");
 			assertThat(plugin).isInstanceOf(StandardGradlePlugin.class);
 			assertThat(((StandardGradlePlugin) plugin).getVersion()).isEqualTo("1.0");
 			assertThat(plugin.isApply()).isFalse();
 		});
+	}
 
+	@Test
+	void addPluginSeveralTimeReuseConfiguration() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		pluginContainer.add("com.example", (plugin) -> {
+			assertThat(plugin.getVersion()).isNull();
+			plugin.setVersion("1.0");
+		});
+		pluginContainer.add("com.example", (plugin) -> {
+			assertThat(plugin.getVersion()).isEqualTo("1.0");
+			plugin.setVersion("2.0");
+		});
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
+			assertThat(plugin.getId()).isEqualTo("com.example");
+			assertThat(plugin).isInstanceOf(StandardGradlePlugin.class);
+			assertThat(((StandardGradlePlugin) plugin).getVersion()).isEqualTo("2.0");
+			assertThat(plugin.isApply()).isFalse();
+		});
 	}
 
 	@Test
 	void applyGradlePlugin() {
 		GradlePluginContainer pluginContainer = new GradlePluginContainer();
 		pluginContainer.apply("com.example");
-		assertThat(pluginContainer.values()).hasSize(1);
-		pluginContainer.values().findFirst().ifPresent((plugin) -> {
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
+			assertThat(plugin.getId()).isEqualTo("com.example");
+			assertThat(plugin.isApply()).isTrue();
+		});
+	}
+
+	@Test
+	void applyGradlePluginSeveralTimesApplyOnlyOnce() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		pluginContainer.apply("com.example");
+		pluginContainer.apply("com.example");
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
 			assertThat(plugin.getId()).isEqualTo("com.example");
 			assertThat(plugin.isApply()).isTrue();
 		});
@@ -68,8 +95,7 @@ public class GradlePluginContainerTests {
 		GradlePluginContainer pluginContainer = new GradlePluginContainer();
 		pluginContainer.add("com.example");
 		pluginContainer.apply("com.example");
-		assertThat(pluginContainer.values()).hasSize(1);
-		pluginContainer.values().findFirst().ifPresent((plugin) -> {
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
 			assertThat(plugin.getId()).isEqualTo("com.example");
 			assertThat(plugin.isApply()).isFalse();
 		});
@@ -80,11 +106,68 @@ public class GradlePluginContainerTests {
 		GradlePluginContainer pluginContainer = new GradlePluginContainer();
 		pluginContainer.apply("com.example");
 		pluginContainer.add("com.example");
-		assertThat(pluginContainer.values()).hasSize(1);
-		pluginContainer.values().findFirst().ifPresent((plugin) -> {
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
 			assertThat(plugin.getId()).isEqualTo("com.example");
 			assertThat(plugin.isApply()).isTrue();
 		});
 	}
 
+	@Test
+	void isEmptyWithEmptyContainer() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		assertThat(pluginContainer.isEmpty()).isTrue();
+	}
+
+	@Test
+	void isEmptyWithRegisteredPlugin() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		pluginContainer.add("com.example");
+		assertThat(pluginContainer.isEmpty()).isFalse();
+	}
+
+	@Test
+	void hasPluginWithMatchingStandardPlugin() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		pluginContainer.add("com.example");
+		assertThat(pluginContainer.has("com.example")).isTrue();
+	}
+
+	@Test
+	void hasPluginWithMatchingAppliedPlugin() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		pluginContainer.apply("com.example");
+		assertThat(pluginContainer.has("com.example")).isTrue();
+	}
+
+	@Test
+	void hasPluginWithNonMatchingPlugin() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		pluginContainer.apply("com.example");
+		assertThat(pluginContainer.has("com.example.another")).isFalse();
+	}
+
+	@Test
+	void removeWithMatchingStandardPlugin() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		pluginContainer.add("com.example");
+		assertThat(pluginContainer.remove("com.example")).isTrue();
+		assertThat(pluginContainer.isEmpty()).isTrue();
+	}
+
+	@Test
+	void removeWithMatchingAppliedPlugin() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		pluginContainer.apply("com.example");
+		assertThat(pluginContainer.remove("com.example")).isTrue();
+		assertThat(pluginContainer.isEmpty()).isTrue();
+	}
+
+	@Test
+	void removeWithNonMatchingPlugin() {
+		GradlePluginContainer pluginContainer = new GradlePluginContainer();
+		pluginContainer.apply("com.example");
+		assertThat(pluginContainer.remove("com.example.another")).isFalse();
+		assertThat(pluginContainer.isEmpty()).isFalse();
+	}
+
 }

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/gradle/GroovyDslGradleBuildWriterTests.java

@@ -89,8 +89,7 @@ class GroovyDslGradleBuildWriterTests {
 	@Test
 	void gradleBuildWithPluginAndVersion() throws IOException {
 		GradleBuild build = new GradleBuild();
-		build.plugins().add("org.springframework.boot",
-				(StandardGradlePlugin standardGradlePlugin) -> standardGradlePlugin.setVersion("2.1.0.RELEASE"));
+		build.plugins().add("org.springframework.boot", (plugin) -> plugin.setVersion("2.1.0.RELEASE"));
 		List<String> lines = generateBuild(build);
 		assertThat(lines).containsSequence("plugins {", "    id 'org.springframework.boot' version '2.1.0.RELEASE'",
 				"}");

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/gradle/KotlinDslGradleBuildWriterTests.java

@@ -91,10 +91,8 @@ class KotlinDslGradleBuildWriterTests {
 	@Test
 	void gradleBuildWithKotlinPluginAndVersion() throws IOException {
 		GradleBuild build = new GradleBuild();
-		build.plugins().add("org.jetbrains.kotlin.jvm",
-				(StandardGradlePlugin standardGradlePlugin) -> standardGradlePlugin.setVersion("1.3.21"));
-		build.plugins().add("org.jetbrains.kotlin.plugin.spring",
-				(StandardGradlePlugin standardGradlePlugin) -> standardGradlePlugin.setVersion("1.3.21"));
+		build.plugins().add("org.jetbrains.kotlin.jvm", (plugin) -> plugin.setVersion("1.3.21"));
+		build.plugins().add("org.jetbrains.kotlin.plugin.spring", (plugin) -> plugin.setVersion("1.3.21"));
 		List<String> lines = generateBuild(build);
 		assertThat(lines).containsSequence("plugins {", "    kotlin(\"jvm\") version \"1.3.21\"",
 				"    kotlin(\"plugin.spring\") version \"1.3.21\"", "}");
@@ -103,8 +101,7 @@ class KotlinDslGradleBuildWriterTests {
 	@Test
 	void gradleBuildWithPluginAndVersion() throws IOException {
 		GradleBuild build = new GradleBuild();
-		build.plugins().add("org.springframework.boot",
-				(StandardGradlePlugin standardGradlePlugin) -> standardGradlePlugin.setVersion("2.1.0.RELEASE"));
+		build.plugins().add("org.springframework.boot", (plugin) -> plugin.setVersion("2.1.0.RELEASE"));
 		List<String> lines = generateBuild(build);
 		assertThat(lines).containsSequence("plugins {",
 				"    id(\"org.springframework.boot\") version \"2.1.0.RELEASE\"", "}");

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/maven/MavenBuildTests.java

@@ -33,8 +33,7 @@ class MavenBuildTests {
 		MavenBuild build = new MavenBuild();
 		build.plugins().add("com.example", "test-plugin",
 				(plugin) -> plugin.execution("first", (first) -> first.goal("run-this")));
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((testPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((testPlugin) -> {
 			assertThat(testPlugin.getGroupId()).isEqualTo("com.example");
 			assertThat(testPlugin.getArtifactId()).isEqualTo("test-plugin");
 			assertThat(testPlugin.getVersion()).isNull();
@@ -49,8 +48,7 @@ class MavenBuildTests {
 		MavenBuild build = new MavenBuild();
 		build.plugins().add("com.example", "test-plugin");
 		build.plugins().add("com.example", "test-plugin", (plugin) -> plugin.setVersion("1.0.0"));
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((testPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((testPlugin) -> {
 			assertThat(testPlugin.getGroupId()).isEqualTo("com.example");
 			assertThat(testPlugin.getArtifactId()).isEqualTo("test-plugin");
 			assertThat(testPlugin.getVersion()).isEqualTo("1.0.0");
@@ -63,8 +61,7 @@ class MavenBuildTests {
 		MavenBuild build = new MavenBuild();
 		build.plugins().add("com.example", "test-plugin", (plugin) -> plugin.setVersion("1.0.0"));
 		build.plugins().add("com.example", "test-plugin", (plugin) -> plugin.setVersion(null));
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((testPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((testPlugin) -> {
 			assertThat(testPlugin.getGroupId()).isEqualTo("com.example");
 			assertThat(testPlugin.getArtifactId()).isEqualTo("test-plugin");
 			assertThat(testPlugin.getVersion()).isNull();
@@ -76,22 +73,13 @@ class MavenBuildTests {
 		MavenBuild build = new MavenBuild();
 		build.plugins().add("com.example", "test-plugin", (plugin) -> plugin.setVersion("1.0.0"));
 		build.plugins().add("com.example", "test-plugin");
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((testPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((testPlugin) -> {
 			assertThat(testPlugin.getGroupId()).isEqualTo("com.example");
 			assertThat(testPlugin.getArtifactId()).isEqualTo("test-plugin");
 			assertThat(testPlugin.getVersion()).isEqualTo("1.0.0");
 		});
 	}
 
-	@Test
-	void mavenPluginCanBeRemoved() {
-		MavenBuild build = new MavenBuild();
-		build.plugins().add("com.example", "test-plugin");
-		build.plugins().remove("com.example", "test-plugin");
-		assertThat(build.plugins().values()).hasSize(0);
-	}
-
 	@Test
 	void mavenPluginExecutionCanBeAmended() {
 		MavenBuild build = new MavenBuild();
@@ -99,8 +87,7 @@ class MavenBuildTests {
 				(plugin) -> plugin.execution("first", (first) -> first.goal("run-this")));
 		build.plugins().add("com.example", "test-plugin",
 				(plugin) -> plugin.execution("first", (first) -> first.goal("run-that")));
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((testPlugin) -> {
+		assertThat(build.plugins().values()).hasOnlyOneElementSatisfying((testPlugin) -> {
 			assertThat(testPlugin.getExecutions()).hasSize(1);
 			assertThat(testPlugin.getExecutions().get(0).getId()).isEqualTo("first");
 			assertThat(testPlugin.getExecutions().get(0).getGoals()).containsExactly("run-this", "run-that");
@@ -111,16 +98,16 @@ class MavenBuildTests {
 	void mavenPluginExtensionsNotLoadedByDefault() {
 		MavenBuild build = new MavenBuild();
 		build.plugins().add("com.example", "test-plugin");
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((testPlugin) -> assertThat(testPlugin.isExtensions()).isFalse());
+		assertThat(build.plugins().values())
+				.hasOnlyOneElementSatisfying((testPlugin) -> assertThat(testPlugin.isExtensions()).isFalse());
 	}
 
 	@Test
 	void mavenPluginExtensionsCanBeLoaded() {
 		MavenBuild build = new MavenBuild();
-		build.plugins().add("com.example", "test-plugin", (plugin) -> plugin.extensions());
-		assertThat(build.plugins().values()).hasSize(1);
-		build.plugins().values().findFirst().ifPresent((testPlugin) -> assertThat(testPlugin.isExtensions()).isTrue());
+		build.plugins().add("com.example", "test-plugin", MavenPlugin::extensions);
+		assertThat(build.plugins().values())
+				.hasOnlyOneElementSatisfying((testPlugin) -> assertThat(testPlugin.isExtensions()).isTrue());
 	}
 
 }

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/maven/MavenBuildWriterTests.java

@@ -436,7 +436,7 @@ class MavenBuildWriterTests {
 		MavenBuild build = new MavenBuild();
 		build.setGroup("com.example.demo");
 		build.setArtifact("demo");
-		build.plugins().add("com.example.demo", "demo-plugin", (plugin) -> plugin.extensions());
+		build.plugins().add("com.example.demo", "demo-plugin", MavenPlugin::extensions);
 		generatePom(build, (pom) -> {
 			NodeAssert plugin = pom.nodeAtPath("/project/build/plugins/plugin");
 			assertThat(plugin).textAtPath("groupId").isEqualTo("com.example.demo");

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/maven/MavenPluginContainerTests.java

@@ -24,15 +24,15 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link MavenPluginContainer}.
  *
  * @author HaiTao Zhang
+ * @author Stephane Nicoll
  */
 public class MavenPluginContainerTests {
 
 	@Test
-	void addMavenPlugin() {
+	void addPlugin() {
 		MavenPluginContainer pluginContainer = new MavenPluginContainer();
 		pluginContainer.add("com.example", "test-plugin");
-		assertThat(pluginContainer.values()).hasSize(1);
-		pluginContainer.values().findFirst().ifPresent((plugin) -> {
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
 			assertThat(plugin.getGroupId()).isEqualTo("com.example");
 			assertThat(plugin.getArtifactId()).isEqualTo("test-plugin");
 			assertThat(plugin.getVersion()).isNull();
@@ -40,14 +40,13 @@ public class MavenPluginContainerTests {
 	}
 
 	@Test
-	void addMavenPluginWithConsumer() {
+	void addPluginWithConsumer() {
 		MavenPluginContainer pluginContainer = new MavenPluginContainer();
 		pluginContainer.add("com.example", "test-plugin", (plugin) -> {
 			plugin.setVersion("1.0");
 			plugin.execution("first", (first) -> first.goal("run-this"));
 		});
-		assertThat(pluginContainer.values()).hasSize(1);
-		pluginContainer.values().findFirst().ifPresent((plugin) -> {
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
 			assertThat(plugin.getGroupId()).isEqualTo("com.example");
 			assertThat(plugin.getArtifactId()).isEqualTo("test-plugin");
 			assertThat(plugin.getVersion()).isEqualTo("1.0");
@@ -57,4 +56,65 @@ public class MavenPluginContainerTests {
 		});
 	}
 
+	@Test
+	void addPluginSeveralTimeReuseConfiguration() {
+		MavenPluginContainer pluginContainer = new MavenPluginContainer();
+		pluginContainer.add("com.example", "test-plugin", (plugin) -> {
+			assertThat(plugin.getVersion()).isNull();
+			plugin.setVersion("1.0");
+		});
+		pluginContainer.add("com.example", "test-plugin", (plugin) -> {
+			assertThat(plugin.getVersion()).isEqualTo("1.0");
+			plugin.setVersion("2.0");
+		});
+		assertThat(pluginContainer.values()).hasOnlyOneElementSatisfying((plugin) -> {
+			assertThat(plugin.getGroupId()).isEqualTo("com.example");
+			assertThat(plugin.getArtifactId()).isEqualTo("test-plugin");
+			assertThat(plugin.getVersion()).isEqualTo("2.0");
+		});
+	}
+
+	@Test
+	void isEmptyWithEmptyContainer() {
+		MavenPluginContainer pluginContainer = new MavenPluginContainer();
+		assertThat(pluginContainer.isEmpty()).isTrue();
+	}
+
+	@Test
+	void isEmptyWithRegisteredPlugin() {
+		MavenPluginContainer pluginContainer = new MavenPluginContainer();
+		pluginContainer.add("com.example", "test-plugin");
+		assertThat(pluginContainer.isEmpty()).isFalse();
+	}
+
+	@Test
+	void hasPluginWithMatchingPlugin() {
+		MavenPluginContainer pluginContainer = new MavenPluginContainer();
+		pluginContainer.add("com.example", "test-plugin");
+		assertThat(pluginContainer.has("com.example", "test-plugin")).isTrue();
+	}
+
+	@Test
+	void hasPluginWithNonMatchingPlugin() {
+		MavenPluginContainer pluginContainer = new MavenPluginContainer();
+		pluginContainer.add("com.example", "test-plugin");
+		assertThat(pluginContainer.has("com.example", "another-plugin")).isFalse();
+	}
+
+	@Test
+	void removeWithMatchingPlugin() {
+		MavenPluginContainer pluginContainer = new MavenPluginContainer();
+		pluginContainer.add("com.example", "test-plugin");
+		assertThat(pluginContainer.remove("com.example", "test-plugin")).isTrue();
+		assertThat(pluginContainer.isEmpty()).isTrue();
+	}
+
+	@Test
+	void removeWithNonMatchingPlugin() {
+		MavenPluginContainer pluginContainer = new MavenPluginContainer();
+		pluginContainer.add("com.example", "test-plugin");
+		assertThat(pluginContainer.remove("com.example", "another-plugin")).isFalse();
+		assertThat(pluginContainer.isEmpty()).isFalse();
+	}
+
 }

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/maven/MavenPluginTests.java

@@ -57,13 +57,11 @@ class MavenPluginTests {
 	@SuppressWarnings("unchecked")
 	void configurationParameterWithNestedValuesCanBeCustomized() {
 		MavenPlugin plugin = new MavenPlugin("com.example", "test-plugin");
-		plugin.configuration(
-				(configuration) -> configuration.configure("plugins", (items) -> items.add("item", "one")));
-		plugin.configuration(
-				(configuration) -> configuration.configure("plugins", (items) -> items.add("item", "two")));
+		plugin.configuration((configuration) -> configuration.configure("items", (items) -> items.add("item", "one")));
+		plugin.configuration((configuration) -> configuration.configure("items", (items) -> items.add("item", "two")));
 		assertThat(plugin.getConfiguration().getSettings()).hasSize(1);
 		Setting setting = plugin.getConfiguration().getSettings().get(0);
-		assertThat(setting.getName()).isEqualTo("plugins");
+		assertThat(setting.getName()).isEqualTo("items");
 		assertThat(setting.getValue()).isInstanceOf(List.class);
 		List<Setting> values = (List<Setting>) setting.getValue();
 		assertThat(values.stream().map(Setting::getName)).containsExactly("item", "item");
@@ -74,13 +72,13 @@ class MavenPluginTests {
 	@SuppressWarnings("unchecked")
 	void configurationParameterWithSeveralLevelOfNestedValuesCanBeCustomized() {
 		MavenPlugin plugin = new MavenPlugin("com.example", "test-plugin");
-		plugin.configuration((configuration) -> configuration.configure("plugins",
+		plugin.configuration((configuration) -> configuration.configure("items",
 				(items) -> items.configure("item", (subItems) -> subItems.add("subItem", "one"))));
-		plugin.configuration((configuration) -> configuration.configure("plugins", (items) -> items.configure("item",
+		plugin.configuration((configuration) -> configuration.configure("items", (items) -> items.configure("item",
 				(subItems) -> subItems.add("subItem", "two").add("subItem", "three"))));
 		assertThat(plugin.getConfiguration().getSettings()).hasSize(1);
 		Setting setting = plugin.getConfiguration().getSettings().get(0);
-		assertThat(setting.getName()).isEqualTo("plugins");
+		assertThat(setting.getName()).isEqualTo("items");
 		assertThat(setting.getValue()).isInstanceOf(List.class);
 		List<Setting> items = (List<Setting>) setting.getValue();
 		assertThat(items).hasSize(1);